The Cornell Lab of Ornithology-Bird Populations?

Bird populations fluctuate seasonally and from one year to the next for a range of reasons. Often when someone reports that birds have gone missing from their yard, they are just seeing normal variation. Causes for these regular changes include:

Fluctuating food supplies/requirements. Cones, berries, seeds, and insects change from year to year, causing birds to move about to take advantage of food surpluses and to escape from areas with food shortages. Also, birds have different dietary needs during different times of the year, so they may move to or away from your feeders seasonally.

Predator populations. Foxes, birds of prey, cats, and other predators have fluctuating populations too. When their populations are high, bird populations may fall. This can also happen on a very local scale, such when a hawk takes up residence in your yard. When the predators move on, your birds will come back. Here’s what to do about a hawk in your backyard.

Disease. On rare occasions, outbreaks of diseases can sharply reduce numbers of certain birds. Examples include the effect of West Nile virus on crows in the early 2000s; House Finch eye disease; and salmonellosis on feeder birds. Learn more about diseases and how to keep your feeders clean.

Habitat change. Tree removal, housing developments, land clearing, fires, and other changes can change the number or types of birds you see.

On the other hand, we know that many populations of bird species are in fact declining consistently from year to year. The North American Breeding Bird Survey estimates how much species’ populations have changed since the mid-1960s. In 2010, Partners In Flight compiled a list of Common Birds in Steep Decline—42 species that have lost 50% or more of their population since the 1960s. You can also refer to their full database on conservation status of North American landbirds.